This invention relates generally to compositions useful in fabricating blocks or inserts for inductors, and more particularly to compositions useful in fabricating blocks or inserts for inductors used in induction heating. The invention also relates to the inductor manufactured using such blocks or inserts, the method of making the composition and the inductor, and the method of employing the inductor in the heat-treatment of workpieces.
Inductors or inductor coils are generally used to heat conductive material by currents induced by varying an electromagnetic field. Electromagnetic energy is transferred from the inductor to a workpiece. For purposes of analogy, if the inductor coil is considered to be the primary winding of a transformer, then the workpiece which is about to be heated would be considered the single-turn secondary. When an alternating current flows in the primary coil or inductor, secondary current will be induced in the workpiece. These induced currents are called eddy currents and the current flowing in the workpiece can be considered as the summation of all of the eddy currents. Heat is generated in the workpiece by hysteresis and eddy current losses, with the heat generated being a result of the energy expended in vvercoming the electrical resistance of the workpiece. Typically, close spacing is used between the inductor coil and the workpiece, and high coil currents are used to obtain maximum induced eddy currents and resulting high heating rates.
Induction heating is widely employed in the metal working industry to heat metals for soldering, brazing, annealing, hardening, forging, induction melting and sintering, as well as for other various induction heating applications. As compared to other conventional processes, induction heating has several inherent advantages. First, heating is induced directly into the material. It is therefore an extremely rapid method of heating. It is not limited by the relatively slow rate of heat diffusion in conventional processes using surface contact or radiant heating methods. Second, because of a skin effect, heating is localized and the area of the workpiece to be heated is determined by the shape and size of the inductor coil. Third, induction heating is easily controllable, resulting in uniform high quality of the product. Fourth, induction heating lends itself to automation, in-line processing, and automatic process cycle control. Fifth, start-up time is short, and thus standby losses are low or nonexistent. And sixth, working conditions are better because of the absence of noise, fumes, and radiated heat. Of course, there are also other advantages.
It is well known that the magnetic flux generated by the inductor must be dense enough to bring the workpiece to a desired temperature in a specified time (typically short). When the workpiece is simple in shape and can easily be surrounded by the inductor, rapid heating using a conventional inductor is a relatively simple task. However, when the workpiece is of a more complex shape, it becomes difficult to assure rapid and uniform heating in areas which are not readily accessible to the inductor.
In the past, it has been recognized that the performance of inductors may be improved by controlling the direction of flux flow and thereby manipulating and maximizing flux density on the workpiece. For example, with an inductor coil of generally circular cross-section, directional control might be improved by attaching magnetic field orienting elements on certain portions of the circumference, so that flux is intensified on the corresponding area of the workpiece. Presently used field orienting elements include laminations made of grain-oriented iron (which are relatively thin pieces of strip stock) which are attached to the inductor on a strip by strip or layer by layer basis as necessary. These laminations, however, are unsatisfactory to the extent that they are difficult to apply, requiring cutting and sizing to the necessary configuration. Thus limited portions or parts of an inductor cannot be covered because of the difficulty of application. In this regard, it is very tedious and difficult to laminate such strip stock on to complicated geometrical shapes of the type which are often needed to heat-treat certain types of workpieces. Applying such laminations to large inductors is also somewhat prohibitive due primarily to cost and labor considerations. In addition, these iron laminations have a tendency to lose permeability at high operating temperatures. This results in inefficient heat treating operations. At high temperatures, these materials require cooling due to relatively high hysteresis and eddy current losses. Laminations made of grain-oriented iron are also relatively expensive due to labor cost required for manufacture.
Another conventional method of controlling the direction of inductor flux density is by the use of blocks or inserts made of ferromagnetic material in a binder. Tetraflouroethylene (TFE) polymers have been used as binders in these blocks or inserts.
Ihe present invention relates to novel and improved compositions useful in the fabrication of such blocks or inserts. These compositions employ a high purity, annealed, electrolytically prepared iron powder with a unique physical characteristic and a polymer binder which includes a resin or mixtures of resins. The compositions may optionally employ an additional material or component such as an acid phosphate insulating coating. Inserts or blocks fashioned from the resulting compositions show improved performance when employed in induction heat treatment modalities over conventional, art-disclosed materials in that the insert formed from these ccmpositions maintains the necessary permeability and demonstrates a maximum of about five (5) percent regression in permeability between the commonly employed frequencies of 10 KHz and 500 KHz and a total core loss of less than about 0.8 to about 1.2 ohms in this range. The iron powder in the compositions and methods of the present invention is characterized in that it is substantially non-spherical and generally flat or disc-shaped and possesses a specific surface area of less than about 0.25 m.sup.2 /g. The iron powder described above is particularly suited to use in preparing inserts in that it permits the formation of an insert by pressing at relatively high pressures with the insert possessing a very high density with an extremely high ratio of ferromagnetic material: binder material while still permitting the binder to perform well. The present invention also relates to the method of preparing these compositions; the block or insert prepared from these compositions and the method of preparing them; the inductor manufactured employing these blocks or inserts; and the method of heat-treating a workpiece employing these inductors.